in a normal lung, the overall ventilation/perfusion ratio is approximately 0.8, but the range of V̇ /Q̇ ratios varies widely in different lung units. When ventilation exceeds perfusion, the ventilation/perfusion ratio is greater than 1 (V̇ /Q̇ > 1), and when perfusion exceeds ventilation, the ventilation/ perfusion ratio is less than 1 (V̇ /Q̇ < 1). Mismatching of pulmonary blood flow and ventilation results in impaired O2 and CO2 transfer. In individuals with cardiopulmonary disease, mismatching of pulmonary blood flow and alveolar ventilation is the most frequent cause of systemic arterial hypoxemia (reduced PaO2). In general, V̇ /Q̇ ratios greater than 1 are not associated with hypoxemia.
A normal ventilation/perfusion ratio does not mean that ventilation and perfusion of that lung unit are normal; it simply means that the relationship between ventilation and perfusion is normal. For example, in lobar pneumonia, ventilation to the affected lobe is decreased. If perfusion to this area remains unchanged, perfusion would exceed ventilation; that is, the ventilation/perfusion ratio would be less than 1 (V̇ /Q̇ < 1). However, the decrease in ventilation to this area produces hypoxic vasoconstriction in the pulmonary arterial bed supplying this lobe. This results in a decrease in perfusion to the affected area and a more “normal” ventilation/perfusion ratio. Nonetheless, neither the ventilation nor the perfusion to this area is normal (both are decreased), but the relationship between the two could approach the normal range.
The ventilation/perfusion ratio varies in different areas of the lung. In an upright individual, although both ventilation and perfusion increase from the apex to the base of the lung, the increase in ventilation is less than the increase in blood flow. As a result, the normal V̇ /Q̇ ratio at the apex of the lung is much greater than 1 (ventilation exceeds perfusion), whereas the V̇ /Q̇ ratio at the base of the lung is much less than 1 (perfusion exceeds ventilation). The relationship between ventilation and perfusion from the apex to the base of the lung is depicted in
Fig. 23.7 • Fig. 23.7 Ventilation/Perfusion Relationships in a Normal Lung in the Upright Position. Only the apical and basal values are shown for clarity. In each column, the number on top represents values at the apex of the lung, and the number on the bottom represents values at the base. PCO2, partial pressure of carbon dioxide; PN2, partial pressure of nitrogen; PO2, partial pressure of oxygen; Q̇ , perfusion per minute; VA, alveolar ventilation per minute.
PACO2 and PaCO2 are equal because of the solubility properties of CO2 (see ). The same is not true for alveolar and arterial O2. Even in individuals with normal lungs, PAO2 is slightly greater than PaO2. The difference between PAO2 and PaO2 is called the alveolar-arterial difference for oxygen (AaDO2). An increase in the AaDO2 is a hallmark of abnormal O2 exchange. This small difference in healthy individuals is not caused by “imperfect” gas exchange, but by the small number of veins that bypass the lung and empty directly into the arterial circulation. The thebesian vessels of the left ventricular myocardium drain directly into the left ventricle (rather than into the coronary sinus in the right atrium), and some bronchial and mediastinal veins drain into the pulmonary veins. This results in venous admixture and a decrease in PaO2. (This is an example of an anatomical shunt; see the section .”) Approximately 2% to 3% of the cardiac output is shunted in this way.
To measure the clinical effectiveness of gas exchange in the lung, PaO2 and PaCO2 are measured. PAO2 is calculated from the alveolar air equation (